Spaced wave keying



y Aug- 21, 1945-1 R LEE HOLLINGSWORTH 2,383,126

SPACED WAVE KEYING Filed July 8, `1943 2 Sheets-Sheet 1 INVENToR.

. if HOLL/NGSWRTH BYY )f5/mw A77' ORNE Y AAug. 2l, 1945.

R LEE HOIfLlNGSWORTH SPACED WAVE KEYING Filed July 8, 1943 2 Sheets-Sheet 2 T ro TUNE AAA nu v" vg MDUL TOR INVNTOR.

R. LEEf/oLL//vaswo/zrf/ Patented Aug. 21, 1945 sPAcED WAVE KEYING ,R Lee Hollingsworth, Riverhead, N. Y.,as`signor to Radio Corporation of America, a corporation of Delaware Application July s, 1943, serial No. 493,846

1,8 Claims.

The transmission of mark and space wave keying is not new in principle since itwas one `of the earliest forms of keying in high power radio transmitters. The older method of receiving such mark and space wave keying depended on the selectivity of the receivers' to discriminate against the unwanted `spacing wave. A newer conception utilizing both the mark and `space waves to approXimately double the effective signal strength at the receiver is herein described.

It is one of the objects of the present invention to provide an improved system for receiving mark and space wave or frequency shift keying to accomplish the effective signal increase as men` tioned above.` I

It is another object of thisinvention to provide in combination with such a method of reception. the means for alternately supplying the automatic volume control circuit with rectified mark and space wave energy for automatic gain control purposes.

It is still another object of this invention to provide a mark and space wave receiver having two commonly excited intermediate frequency amplifiers, one to receive thernark wave energy, while the other receives the space waveenergy antennae are shown connected respectively to radio frequency amplifiers 4, 5, and 6. The output of these radio frequency ampliers are connected to detectors and intermediate frequency amplifiers 1, 8 and 9. Each of the radio frequency ampliers have two tuned output circuits shown at I0, II, I2, I3, I4 and I5. These output circuits are shown inductively coupled to tuned circuits I6 and l1, I8 and I9, and 20 and 2|. Eachof the tuned output circuits are shown con- `nected to rectiliers 22, 23, 24, 25, 26 and 21.

and to provide the 'meansior rendering the output of one of the channels non-operative, while the other channel is operative and vice versa, to insure that` the intermediate frequency band width looking into the keyer or translator system shall never be wider than the band width possessed by either one `of the two intermediate frequency amplifying channels.

The description of my invention follows, and

in this description reference is made to the attached drawings in which Fig. 1 shows schematically a diversity receiver for use in radio telegraph and telephone receiving work, andV including the Ilovel features of my means of receiving mark and space wave keying. Fig. 2 is a modification of thearrangementof Fig. 1. e

In Fig. 2, I show my receiver utilizing the principle of mark and` space wave keying together with my improved means for supplying automatic volume control voltage alternately from the mark and space wave energy for simultaneously rendering the space wave output circuit ttally inoperative when mark wave signal energy is being received, and vice versa andfor operating the signal translating or recording circuits.

Referring specically to Fig. 1, the parts are identied as follows: Numerals I, 2 and 3 indicate receiving antennae that are normallyseparated from onelanother by several wave lengths. These Ground potential throughout Fig. 1 is indicated` by numeral 28. 29 represents a direct current meter to indicate the space vwave rectified output energy while 3l!` represents a direct current meter for `indicating the mark wavev energy output. 3| is a terminating resistance for the space Wave current rectiers 23, 25 and 21. Tube 32 is a oneway electron discharge` device with its cathode connected to the upper end of resistance 3I and its anode connected by leads 33 in the automatic volume control circuitsfor the radio frequency ampliers in units 4, 5 and 6 and/or the ampliners in units 1, 8 and 9. ,l Y t 34'is a keying tube having its cathode also connected to the upper endoi resistance 3I and `its anode connected by source 35 and resistance 36 to ground. Source 35 furnishes the plate supply for tube 34 while in resistance 36 is developeda suitable negative blocking bias for the grids of the tone modulator tubes 42 and 43.

31 is a second electron `discharge tube which is, in a sense, a signal keying tube having its control grid` also coupled to the upper end of resistance 3| and its anode connected by source38 and resistance V39 to ground. This tube provides automatic volume control voltage in the absence of` both mark` and space wave energy. A tone supply is coupled to the primary windings 40 of `a transformer, the secondarywinding 4I of which denser 41 comprisesJ direct current lter, serv-` ing as a time constant in the automatic volume control circuit. Similar arrangements have been shown in my U. S. Patent #2,104,324, dated January 4, 1938,A and U. S. Patent #2,243,423, dated May 27, 19.414.

49 is a resistance in the direct current path including the marking current meter 30 and rectiers 22, 24 and 26. Note that the upper end of this resistance 49 is connected through resistor 48 of the AVC filter network to the leads 33, and also to the control grid of the keying tube 34.

In operation let us assume that the carrier frequency assignment of the mark wave is 10,000.500 kilocycles. The energy from the antennae is amplified by radio frequency amplifiers 4, 5 and 6 in the usual manner and passed on to receiver units 1, 8, and 9 where the frequency of the incoming wave signals are changed to desired intermediate frequency using the well known superheterodyne principle. Within units 1, 8 and 9, if desired, a second means of conversion to still a lower intermediate frequency might be employed. Within these receiver units two intermediate frequency channels may be provided for separately amplifying the mark and space wave energies which are finally rectied by rectifiers 22, 23, 24, 25, 26 and 21. On the other hand, the mark and space Wave energy as illustrated in Fig. l is amplified by a common intermediate frequency amplifier having sufficient bandwidth to pass both wave energies and a filter means provided for separating the mark and space wave signals before they are passed to their respective rectiers.

` Let us assume that the final intermediate frequency chosen be 50 kilocycles and that the amplifier or filter system used to pass the mark Wave signal be tuned to this value, and that the intermediate frequency amplifier or lter system used to pass the space wave energy be tuned and centered at 50.500 kilocycles per second. If the transmission to be received has a frequency shiftdisplacement of 500 cycles per second, when the incoming mark wave signal is centered through the intermediate frequency amplifier it would likewise follow that the space wave signal would automatically be centered through the intermediate frequency amplifier 500 cycles removed.

Let us assume that carrier Wave signals are being received and that the transmitter radiation is taking place on the space wave frequency, and that rectifiers 23, 25, and 21 are passing rectified currents through space indicating meter 29 and terminating resistance 3|. Connected above resistance 3| is the one-way electron discharge tube 32 that passes direct current into the AVC supply 33. Also connected to the negative side of resistance 3| is the cathode of keying tube 34. This makes the grid of this tube more positive relative to the cathode and causes this tube to be more than normally conductive and an increased current as a result thereof is drawn by source 35 through resistance 36 during spaces. The negative potential drop developed across resistance 36 is applied to the control grids of the balanced tone modulator tubes 42 and 43 to increase the normalblocking Voltage which is applied to these tubes, thereby preventing noise currents which might occur (during space) from actuating this balanced modulator stage to send rout keyed tone and thereby improperly operate the recording mechanism. Thus, in my novel system, the space wave is dually used to supply automatic volume control voltage at resistance 3| and at the same time to render the keying tube 34 and balanced modulator including tubes 42 and 43 inoperative under conditions when reasonable signals are being received.

If, during this period of idle transmission the space Wave should be removed completely or fade to a very low input level, bias supply tube 31 becomes conductive to supplythe automatic volume control potential. i

Explaining'this action in de-` tail, the control grid of tube 31 is connected to the upper end of resistance 3| so thatas the space Wave signals fade out, the grid of 31 becomes more positive with respect to the grounded cathode of this tube and current is drawn through resistance 39 and source 38, thus supplying a potential drop in resistance 39 which is impressed by way of resistance 46 on the automatic volume control circuit. Thus the tube 31 supplies through resistance 46 an automatic volume control voltage in the absence of, or reduction of, signal energy beyond a usable value.

Let *us now assume that transmission of intelligence begins and that the signal impulses of the mark wave energy now is passed by rectirlers 22,

f virtue of the voltage drop produced by the rectified mark wave signal current across terminating resistance 49. The tube 34 as a consequence is instantaneously biased to cutoff or near cutoff as a result of the application of more negative potential to its lcontrol grid and less negative potential to its cathode. With the current through tube 34 greatly reduced or entirely cutoff the blocking bias developed across resistance 36 is reduced and this allows the control grids of balanced modulator tubes 42 and 43 to be biased normally, thus allowing the tone supply to be amplified and passed to the line 45.

It should be appreciated that in my improved system when space wave energy is being received, if heavy static crashes are present, the instantaneous feeding to the automatic volume control circuit of the potential developed as a result of the said static across the resistance 3| instantaneously decreases the gain of the receiver and simultaneously increases the conductivity of keyer tube 34, This practically precludes the possibility of static crashes actuating the balanced modulator tubes 42 and 43, as a result of the fast AGC action taking place through diode 32, which instantaneously increases the bias on balanced modulators 42 and 43, and which simultaneously knocks the gain of the receiver downward. It is intended that resistance 3| shall be somewhat larger than resistance 49 to insure that this desired control action originating across resistance 3l will always be started in advance of any unwanted action that may otherwise result from noise currents passing through resistance 49. With the loss of space wave energy and the application of mark wave energy the automatic volume control voltage developed in resistance 49 may not be supplied as rapidly. However, if found desirable, the time constant values of resistances 46 and 48 and condenser 41 may be reduced in size to allow, for all practical purposes, the same rate of automatic volume control compensation as is present during the reception of space wave energy. f Y

In the modification described in detail above and illustrated in Fig. 1, I have shown a plurality of receivers in a diversity system. It will be understood, however, that single receivers may be used. Such modification obviously requires merely that the receivers not used are disconl f 2,383,126 l nectedfrom the automatic volume control circuit and from the mark and space direct current paths through space `and mark meters 29 and3. Y

I-n the embodiment illustrated in Fig: 2, a single vreceiver system is shown, comprising an antenna 50 for collecting magnetic wave` energy and feeding the same to a radio frequencyfamplii-ler 5l. The outpu't of 5|; is connected tothe input of a receiver in 52. Within. the 4unit'52 there is included theusual frequency-` converting means as used in superheterodyne receivers to 59V andlresistance (i5 to ground;` As a consequence 'oty this increase in negative potential at .the upper end of resistance B5 and on the` con-` trol grid of tube 60,.the c urrentthrough tube 6|] l is reduced or shut orf so that the potential drop in 62 is `reduced,.thereby greatly reducing or reducing entirely the negative blocking voltage present at 63, which .in normal operation allows change the frequency of the incoming wave energy to the desired intermediate .,frequency.

Thel unit 52 also includes an intermediate fre-` quency amplifier and it is assumedthat two intermediate `frequency vamplifying channelsare present therein as previously explainedin` connection with Fig. l.

frequency, representing markland space. 53

and 54 represent `tuned output circuits coupled to the intermediate frequency amplifying chan-` nels and these tuned output circuits are coupled to triode rectifying tubes 55 and56, respectively, by wayof tuned circuitsv53uand54f.

vIn the description which follows, it is assumed that the rectier `tube 55 passes the spacing wave energyfrom a frequency shift transmitter, and that rectier 56 passes the marking wave energy from said frequency shift transmitter. Throughout this figure 5l represents ground and it will be noted that the cathode of tubes 55: and` 56 are grounded, while the anodes thereof are connected to tuned circuits 53 and 54" and coupled by said` tuned circuits 53" and 5N to` tuned circuits 53 and 54, respectively; One end of the tuned circuits 53 and 54' `are connected as shown to the cathodes of triodcs 58 and 59.`

The anodes of triodes 58 and 59 are connected to ground by resistors 54 and 65, respectively, `so that the mark and spacev wave rectified currents are passed alternately through series triodes 58 and 59 andthrough terminating resistances Ell and 65 to ground and back to the cathodes ofrectiiiers 55 and` 56, respectively.A

The cathode of keying tube'GO' is connected` to` the negative potential end of resistance 94 and during the reception of space wave energy the cathode becomes more negative relative to the control grid `Whichis connected yto the negative terminal of resistance 55, so that this tube `on the reception of space wave energy is rendered more conductive. 0de of tube B0 due tothe action of batteryil draws more current through resistance 62thus. higher `blocking bias atthe point When the space Wave signal energy ceasesto flow through resistance B4, the cathode offkey` ing tube 60 becomes less negative and this tube is presently or instantaneously rendered nearly orcompletely non-conductive with the application of negative potential toits control'A `grid when the marking Wave energy is impressed onl 5'4, rectified in tube "56 and flows throughtube The twochannels include' filters for separating the currents of reduced` As a consequence the an.-`

the 4balanced modulator tubes to become conductive, amplify a tone and supply the same from the `output transformer over lines to the recorder `or translator system. l

`The cathode end of tube 58 is coupled by a one-way tube 61 to the automatic volume control f circuit 68, fwhich-` includes resistance 56 andan adjustable portion ofyresistance 62. The cathode end of tube 59 isalso coupled in a like manner to theautomatic volume control circuit through` the uni-directional device 6l. The control grid of tube 59 and the` control grid of rectifier 56 are-coupled to the-cathode of tube 58, that is, to a point on the space Wave rectier circuit, while thecontrol grid of tube 58 andthe control grid of rectifier 55 are coupled to the cathode of tube 59, ythat is, toa point Son the rectifying circuit for the mar-k Wave energy. l

When the space wave current is present, rectified current passes through rectifier 55, tube 58, resistance 64, and ground, and automatic volume -control potential is supplied through the `unilateral device 6l and at the same time is applied to the controlgrid` of rectifier 59 and thecon-` trol grid of series tube 59 to render Athe marking channel'completely inoperative. This is `because the negative potential at the cathode end of triode58 is applied to the control gridso-f rectifier 56"and` tube` 59, thereby increasing their impedance. Conversely, when the marking wave `signals. are received andrectiiied'in 56 and rectied current flows through 56, tube 59, and resistance 65 to ground and back to the cathode of 5t, automatic Volume control voltages are supplied from the cathode of tube 59 through the unilateral 'device (il` to the automatic volume control circuit.

' The negative potential on the cathode of 59 `which increases inthe presence of mark currents is also supplied to the control `grids of tube 58 and 55 to increase the impedance thereof to render `the spacing wave rectifier path non-conductive.

Thus, the .effective bandwidth of the receiver or rectiiier system looking into the keyer and associated transmitter equipment will never be wider thanthiat band width passed by one of the intermediate -frequency amplifying channels, since during mark the Vspacing channel is eiectively,v

` shut off, while duringspace the marking channel is effectively shut off.

In the modiiication` of Fig. 2, resistance 66 is of suflicient, valuefto allow the required amount of automatic control voltage to be built up and` applied to thefautomatic volume controlled tubes `lby `'way of connector 68.` Here, as in Patent 2,104,324, Fig. 3, resistance 62 supplies AGC voltage inabsence ofjsignal energy as well as blocking bias for balanced tone modulators.

It will lbe understood that the control grids of tubes 58and 59 may be, if desired, connected to the automatic volume control leads 68 instead ci` to the cathodes of tubes 59 and 5B, respectively.

`A very particular point of interest in this in-` vention is the almost total discrimination of static and atmospheric noises except during the actual transmission of signallingenergy and the par-V ticular. reduction of `the effects of atmospheric noises Whenspace wave energy is being received,

since this energy and likewise atmospheric noises that `are received during the time when the space wave circuit is conductive renders the keying tube more conductive causing no change in the translator system.

this impulse flows through the spacing channel there will lbe an equalizing effect across resistance 62 since the space and mark wave channels are actuating tube 60 in oppOsite phase relationship. Thus by adjustment of the-automatic volume control to the space channel noise reduction can be accomplished, and furthermore, this adjustment may be automatic as a function of the relative noise current strengths.

In my system then, I show a new and improved means for and methods of automatic volume control, together with a means and method for rendering automatically the intermediate frequency amplifying channels alternately non-conductive to improve the overall operation of the system from a signal to noise ratio viewpoint by allowing only the band width required to pass the keying components to lbe connected to the keyer and translating system.

Therefore the scope of this invention is to be limited only by the fact and spirit of this specification, and by the claims appended hereto.

I claim:

1. In apparatus of the class described, a source of potential representing one signalling condition, a source of potential representing a second signalling condition, an electron discharge tube having a control grid, and a cathode and having an anode arranged to be coupled to recording means, a connection from one of said courses to the control grid of said tube and a connection from the other of said sources to the cathode of said tube.

2. In apparatus of the class described, means including two grid controlled rectifiers for deriving from wave energy, keyed by signals from one signalling frequency to a second signalling frequency, a first potential representing one of said signalling frequencies and a second potential representing the other of said signalling frequencies, a keying tube having a control grid, and a cathode and having an anode coupled to an output circuit, connections for applying the potential derived by one of said rectifier-s to the control grid of the other rectifier and to the control grid of said keying tube, and connections for applying the potential derived by the otherl of said rectiiiers to the control grid of said one recytiiier andy to the cathode of said keying tube.

3. In apparatus of the class described, a source of current representing one signalling condition, a path including an impedance in series with said first source, a source of current representing a second signalling condition, a path includingan impedance in series with said second source, an electron discharge tube having a control grid, and a cathode and having an anode coupled to an output circuit, connections from one of said impedances to the control grid of said tube and connections from the other of said impedances to the cathode of said tube.

4. In apparatus of the class described, means including two gridY controlled rectiers for. deriving from wave energy, keyed lby signals from one signalling frequency to a second signalling frequency, a first current representing one signalling frequency, and a second current representing the other signalling frequency, a path including an impedance in series with one of said rectiiiers, a path including an impedance in series with the other of said rectifiers, a keying tube having a v control grid, and a cathode and having an anode coupled to an output circuit, connections from one of said impedances to* the control grid of said keying tube, connections from the other of said impedances to the cathode of said keying tube, a connection from one of said impedances to the control grid of one of said rectifiers and a connection from the other vof said impedances to the control grid of the other of said rectifiers.

5. In apparatus of the class described, an amplier of controllable gain responsive to wave energy keyed from a first frequency represen-ting a rst keying condition to a second frequency representing a second keying condition, rectifying means for deriving from said wave energy a first current representing said first signalling condition, and a second current representing said second signalling condition, a circuit including a first impedance wherein said first current flows, a circuit including a second impedance wherein said second current iiows, and a gain control circuit coupling said impedances to said amplifier to control the gain thereof.

6. In apparatus of the class described, an ampliiier of controllable gain responsive to wave energy keyed from a, rst frequency representing a iirst keying condition to a second frequency representing a second keying condition, two rectiiiers for deriving from said wave'energy a first current representing said iirst signalling condition, and a second current representing said second signalling condition, a circuit including a iirst impedance wherein said first current flows, a circuit including a second impedance wherein said second current iiows, a gain control circuit coupling said impedances to said amplifier to control the gain thereof, a keying `tube having a control electrode, a cathode, and an output electrode connected to a load, a connection between the control grid of said tube and one of said impedances and a connection between the cathode of said tube and the other of said impedances.

7. In apparatus of the class described, an amplier of controllable gain responsive to wave energy keyed from a first frequency representing a first signalling condition to a second frequency representing a second signalling condition, two grid controlled rectiers deriving from said wave energy a rst current representing said first signalling condition, and a second current representing said second signalling condition, a circuit for one of said rectifiers including a iirst impedance wherein said first current flows, a circuit for the other of said rectiers including a second impedance wherein said second current flows, a gain control circuit coupling said impedances to said amplifier to control ,the gain thereof, a connection between the impedance in the circuit for one of said rectiiiers and the control grid of the other rectifier, and a connection between the impedance in the circuit'for the other of said rectiiiers and the control grid of said onerectifier.

8. In apparatus of the class described, two rectiers for deriving from wave energy, keyed by signals from one signalling frequency to a second signalling frequency, a rst potential representing one of said signalling frequencies and a second potential representing the other of said `connected to recording l "r'neans, connections in- `cludirig Va grid controlled A:tub'e impedance:` in series with one of said i rectiiiers for applying' the potential derived by' one of said rectiiiers lto -the controllg'ridof saidl keyingtube;connections`in-` cluding axgrid controlled tubefimpedancein series Witlfr-tlrer other `of lsaid''-rectii"i'e'r`-s."for applying the potential f derived.' by the `other of said rectifiers to 'the' cathode fof 'said keying tube; connections for applyingth'e` potential derivfedbyf said one of said rectiers to said second namedgrid signalling `frequencies,uakeying tube lhaving'a i "control grid, fand -a 'cathodeivand having an anode lingisaid cathode toground,` the arrangement beingsuchthat thjezbiasl oni said oontrolgrid rela,- tive to said -cathode becomesless'negative in the presence of onecurrent, more negative'in the :presence of -theother current andremains substantially constantin the event noise voltages cause currents toyflow simultaneously in theseci ond and'third impedances.

i i2; .1n combination a 'diversityireceiving system Vincluding a multiple-antenna'means for co1- Vlecting Wave energy at `several locations, a ymeans l for amplifyingconverting and rectifyingfsepacontrolled impedance and'iconnections for apply- `1 ing\thepotential derived'by'the other of said'r'ectiiiers to said'first named lgridco'ntrol1ed impedance.` i

9. In apparatus of the class described,` two grid controlled rectiers `for deriving -from- Wave` energy, keyed by signa-lsifrom one signalling frei 'quency toasecondvsignalling frequency, afirst otherof said signalling""frequencies,` a `keying `tube having av control ;grid,and a catho'de 'and having an anodeconnecte'dinan output circuit,

' connections 'including a grid controlled tube impeclancein series 'Withonefof said rectiiiers Vfor applying thepotential derived by one of` saidrec- `tiersto the "control gridof said `keying tube, connectionsincluding` a gridfcontrolled" tube im- `p'edance'in'series'vviththe= other of said rectiers I:for applyingthe'potentialderiyed by theother of said rectifiers to thezcathode ofisaidjkeying tube, connections f for applying the "potential derived by saidone4 of'saidrectiersitorsaid'second i named `grid controlled fim'pedan'ce, connections for applying theipotential deriv'ed'byithe` otherof v said rectiiiers to said `iirst 'frianredr:grid` controlled i impedance, connections fori-applyingthe potential `derived by said one 'of `saidrectiiie1sto "the control gridof said otherrectifler,A 'and connections `for applying the ,potential derivedbyixsaid other of said rectiiiers l\t`o` thei control-grid of said one of saidlrectiiiers. l c

l0. `In signallingapparatus Jto `be` used to `derive the signals yfrom `Wave energy comprising a `rlrst frequency` representing 1. pace `fand l1a i `second frequency representing niark, `with theenergy `keyedlfron'i mark to spaceor1viceversa,"a iirst rectifierin a direct current, pathincluding animpedance, a second rectifier in aidirectcurrent path including an impedance;V a variablegain amplier lfor -feeding matking Wave energy to one rectier and spacing Wave energy to the other rectifier, connections excited by current inone of said paths for increasing the impedance of the r other path, connections excited by current in said other path for increasing the impedance of said one path, and a gain control circuit coupling both paths to said ampliiier. i i i 11. In a balanced keying system responsive to wherein the other of said currents flows connectratelyithe Wave energies collected byisaidi'nultip'leantenna meansga illter` system preceding each `final rectiiier inthe receivingsystem to cause the marking energy offa frequencyxshifttransl initter towbe rectiii'ed by `one rectifrerwh-ile the spacingenergy from saidfrequency shift'transpotential representin'goneof saidsignalling frerquencies yand `aseco'nd potential representing `the mitter` is rectified `by another rectifier,` a means of combining separately the mark and space energies, a reversing'electron discharge tuberen-` dered more conductivefby` the presence ofi spacing energy'ito increase 'the blockingrbias to an 'associatedgtone relaying `means, #said reversing electron discharge'tubeibeing biasedito or near 'cutoff by the loss of'spacing energy andthe applii i cation of `marking l energy, yan' automatic xvolume *control `system thatr'controls instantaneously -the gain'of the receiversthroughia one-way electron discharge tube during'spacing, said'automaticvolurnecontrol system also'controllin'g the gain of the receivers t-hroughadrect current'lter during marking, andra` second reversing tube to sup- 4plyafbiasto the receivers in the'fabsen'ceof or during a fade of the *marking `andspacing signal energy. i l

fle. 11n combinations diversity receivingsystem including a multipleantenna'means'for collecting electromagnetic `Wave energy lat'several 1orcations, :.Va means for l amplifying 'the incoming i electromagnetic waveenergy; a `means forchans- .ing the frequencyofl the incoming electromagnetic Wave energy to a desired intermediate irequency, a two-channel intermediate frequency yl amplifier contained in 'eachrec'eiver of 'said diversity receiving system, separate -rectiiiers "for rectifying the electromagneti'c'wave energy from i each* intermediate'frequency amplier channelto cause `thej'marking. energy of a r frequency shift transmittertobe ampliiied and rectified by one signal channelwhile the spacingenergyfrom said `frequency shift ltransnn'ter is ampliedand rectiyiied by another signal channel, a, means-forcem- "biningjseparately `the-mark and space energies, i a reversing electron discharge tube rendered more conductive by the presence of spacing wave energy to increase the blocking bias to an associated tone relaying means, said reversing electron discharge tube biased to or near cutoff by the loss of space Wave energy and the application of marking Wave energy, an associated automatic Volume control system that controls instantaneously the gain of thereceivers through a one-Way electron discharge tube during spacing, said automatic volume control system also controlling the gain of the receivers through a direct current iilter during marking,` and a second reversing tube to supply a bias to the receivers in the absence of or during a fade of marking and spacing signal energy.

14.` Aradio receiving system comprising a, rst Wave amplifying means, a frequency conversion means, aisecond `Wave amplifying means commonly excited but possessing two separate wave signal channels, a rectier to vproduce direct current from each wave signal channel said direct current disposed to pass through a series rectifier and a terminating resistance, an associated reversing tube made more conductive as a result of one signal channel delivering space wave signal energy from a frequency shift transmitter, said reversing tube made less conductive with the loss of space wave signal energy and rendered nonconductive with the application of marking Wave signal energy from said frequency shift transmitter to operate an associated tone keyer, said reversing tube also supplying an" auxiliary automatic volume control voltage during the absence of or great reduction of marking and spacing wave energy, an automatic volume control system which derives its control voltage alternately from the mark and space signal voltages through two rectiiiers utilizing aforesaid auxiliary automatic Volume control voltage in the absence of or great reduction of both mark and space signal voltages, a means of rendering the spacing Wave channel inoperative when mark wave signal energy is being received and vice Versa.

15. In apparatus of class described, a variable gain amplifier for amplifying wave energy keyed in accordance with signals from a first frequency representing space to a second frequency representing mark and vice versa, a rectifier excited by energy representing space signals for producing direct current in the presence of space signals, a second rectifier excited by energy representing mark for producing direct current in the presence of mark signals, an impedance in series with each rectifier, an automatic volume control cir-cuit coupling each of said impedances to said variable gain ampler for controlling the gain of the amplier in the presence of mark and space signals, and a gain control tube having input electrodes associated with' that impedance in series with the first named rectifier and having an output coupled with said automatic volume control circuit for controlling the gain of said Variable gain amplifier in the presence of space signals of reduced or unusable value.

16. In apparatus of class described, a variable gain amplifier for amplifying wave energy keyed in accordance with signals from a first frequency representing space to a second frequency representing mark and vice versa, a rectifier excited by energy representing space for producing direct current in the presence of space signals, a second rectifier excited by energy representing mark for producing direct current in the presence of mark signals, a keying tube coupled with at least one of said rectiers and controlled by rectified current therein for producing mark and space potentials for recording purposes, an automatic volume' 'with said automatic volume control circuit for controlling the gain of said variable gain amplifier in the presence of wave energy of reduced unusable value.

17. In a signal detecting system, an amplifier of variable gain for amplifying wave energy keyed from a first frequency representing space to a second frequency representing mark and vice versa, a rectifier excited by the wave' energy for producing direct current in the presence of space signals, a second rectifier excitedby the wave energy for producing direct current in the presence of mark signals, a keying tube coupled with both of said rectiers and -controlled by direct current iiowing therein for producing mark and space potentials for recording purposes, an automatic volume control circuit coupling each of said rectifiers to said variable gain amplifier for controlling the gain of the amplifier in the presence of mark and space signals, and a second control tube having electrodes associated with said first named rectifier and havirg'an output coupled with said automatic volume control circuits for controlling the gain of said amplifier in the absence of wave energy of usable strength.

18. In apparatus of the class described, an amplifier of controllable gain responsive to wave energy keyed from a first frequency representing a first keying condition to a second frequency representing a second keying condition, two rectiiiers for deriving from said wave energy a first current representing said first signalling condition, and a second current representing said seoond signalling condition, a, circuit including a first; impedance wherein said first current ows, a circuit including a second impedance wherein said second current ows, a gain control circuit coupling said impedances to said amplifier to control the gain thereof, and a bias supply tube having a control grid coupled to one of said first two impedances and 'having an output electrode in a circuit including an impedance, and connections including some at least of said last named impedance in said gain control circuit.

R LEE HOLLINGSWORTH. 

